1. Field of the Invention
The present invention relates to a fixing device for an image forming apparatus mounted in an image forming apparatus such as a copier, printer, and facsimile for heating and fixing toner images.
2. Description of the Background
As a fixing device used for an image forming apparatus such as an electrophotographic copier and printer, there is a fixing device for inserting sheet paper through a nip formed between a heat roller and a pressure roller and heating, pressurizing, and fixing toner images. Recent years, as a heat-type fixing device, there has been a device in which a metal conductive layer is provided in a heat roller and the metal conductive layer is heated by an induction heating method. The induction heating method is to heat the heat roller by supplying predetermined power to an induction heating coil to generate a magnetic field and instantaneously heating the metal conductive layer with eddy current produced in the metal conductive layer by the magnetic field. In such a heat roller, sometimes an elastic layer is provided outside of a metal core material of the heat roller and the surface of the elastic layer is covered by the metal conductive layer in order to secure a nip width required for fixing between the heat roller and the pressure roller. The elastic layer of the heat roller is made of foamed rubber formed by foaming a silicon rubber material, sponge, or the like, and deforms by the pressure of the pressure roller to form the nip.
However, in the case where the elastic layer is provided between the core material and the metal conductive layer of the heat roller, the coefficient of thermal expansion of the elastic layer such as sponge having fine bubbles is higher than the coefficient of thermal expansion of the metal conductive layer. Accordingly, when the heat roller is heated, the hardness of the heat roller becomes nonuniform in the longitudinal direction thereof due to the difference in coefficient of thermal expansion between the elastic layer and the metal conductive layer. The nonuniformity of hardness of the heat roller in the longitudinal direction causes changes in nip width and heat roller shape and adversely affects the fixing property.
In order to avoid this, conventionally, the elastic layer is formed in a dumbbell shape and the outer diameter of the center part is made smaller than the outer diameters of the both side parts in the longitudinal direction. Thereby, in the central part in the longitudinal direction of the heat roller, space is provided between the elastic layer and the metal conductive layer. Because of the space, the metal conductive layer is prevented from being pushed up from inside by the thermal expansion of the elastic layer when the heat roller is heated, and the hardness of the heat roller in the longitudinal direction is held uniform.
However, in the case where the elastic layer is formed in a dumbbell shape, no load of pressure roller is applied to the central part of the heat roller until the heat roller reaches warm-up temperature. Accordingly, the load due to contact with the pressure roller concentrates on both side parts of the heat roller until the heat roller reaches warm-up temperature. In addition, the elastic layer made of foamed rubber, sponge, or the like is lower in strength than metal cores. Accordingly, there is a possibility that, when the load by pressure of the pressure roller is applied to the both side parts of the heat roller, the elastic layer having lower strength is broken at the boundary part between the core material and the elastic layer and the life of the heat roller becomes shorter.
Therefore, development of a fixing device for an image forming apparatus is desired, in a fixing device for heating and fixing by a heat roller in which an elastic layer is provided around a core material and the surface thereof is covered by a metal conductive layer, a good fixing property can be obtained by holding the hardness of the heat roller in the longitudinal direction uniform and a longer life of the heat roller can be obtained by preventing breakage of the elastic layer at the boundary part between the core material and the elastic layer regardless of pressure contact with a pressure roller.